This invention relates to nuclear reactors and, more particularly, to a fuel bundle for a boiling-water nuclear reactor. A major objective of the present invention is to provide for greater power output for a boiling water reactor of a given size.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
To facilitate handling, fissile fuel is typically maintained in fuel elements. Typically, these fuel elements have a corrosion-resistant cladding. The fuel elements can be grouped together at fixed distances from each other in a fuel bundle. The fuel bundles include spacer grids to maintain alignment and spacing of the fuel bundles. A sufficient number of these fuel bundles are combined to form a reactor core capable of a self-sustaining chain reaction. Neutron-absorbing control rods are inserted into the core to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rods.
In a boiling-water reactor (BWR), heat generated in the core is transferred by water flowing up through the core. Some of the water is converted to steam which can be extracted from the reactor vessel. The extracted steam can be used to drive a turbine, which in turn can drive a generator to produce electricity. Water not converted to steam is recirculated back to the base of the core.
In a BWR, water serves not only as a coolant but also as a moderator. In its role as moderator, the water slows the initially fast neutrons released during fissioning. The slowed or "thermal" neutrons have the appropriate energies for absorption by fissile fuel to produce further fissioning. Steam, because of its lower density, is a much poorer moderator than liquid water. As the water flows up through the core, the percentage of steam increases, so that moderation becomes less effective. Accordingly, some fuel bundles include coolant bypass channels which insulate 1%-2% of the water from the most intense heat generated at the fuel elements. These coolant bypass channels, which are generally in the form of a tube extending from the base to the top of the fuel bundle, provide moderation through the total vertical extent of the bundles. This insures sufficient liquid moderator at all levels within the fuel bundle.
One problem with this bypass approach is that a percentage of the coolant flow is used exclusively for the moderator function. An alternative design uses a convoluted partial height bypass channel. Water flowing up a tube is partially forced into a second interior tube. The outer tube is closed at the top, so water emerging from the top of the interior tube is forced downward and out peripheral holes.
Forced-circulation boiling-water reactors (FCBWRs) use pumps to promote water circulation, while natural-circulation boiling-water reactors (NCBWRs) rely on convection to promote water circulation without pumps. A typical NCBWR employs a chimney over its core to support a driving head. The driving head establishes a pressure differential between the region above the core and the downcomer. The downcomer is the annular space within the reactor vessel to the outside of the core and the chimney. The downcomer defines the path along which water exiting the chimney returns to the core. The pressure differential between the core and chimney on the one hand and the downcomer on the other determines the recirculation rate. The recirculation rate determines the maximum power that can be transferred from the core, and thus the maximum power output of the reactor.
One way to increase the power capability of a NCBWR is to increase chimney height. A taller chimney supports a greater driving head, which in turn supports a greater pressure differential. The resulting increased coolant flow permits more power to be transferred from the core.
However, increasing chimney height requires a larger reactor vessel. A larger reactor vessel requires a larger reactor containment complex. Reactor complex costs and complexity increase geometrically with chimney height. Basic changes, such as increasing chimney height, can only be applied prospectively. Such changes do not address increasing the performance of existing reactors of the forced-circulation type.
What is needed is a design which permits increased power output without increasing reactor size and complexity. This design should be applicable to new NCBWRs. Preferably, the improvement should also be applicable, on a retrofit basis, to enhance the value of existing FCBWRs.